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There has been a long-lasting debate on the pre-onset sequence of substorms. Recently, evidence for plasma flow enhancements in the magnetotail prior to onset has been presented, suggesting magnetic reconnection in the mid-tail plasma sheet preceding substorm onset in the near-Earth plasma sheet. Recently, Nishimura et al. [JGR, 2010] found using the THEMIS all-sky imager (ASI) array that the pre-onset auroral sequence is initiated by a poleward boundary intensification (PBI), which is followed by an approximately north-south (N-S) oriented arc (also called auroral streamer) moving equatorward towards the onset latitude and leading to onset instability in the near-Earth plasma sheet. However, while N-S arcs have been generally associated with fast earthward flows, these are often not associated with a substorm onset. Therefore, the difference between N-S arc sequences that do and do not lead to substorm onset within the near-Earth plasma sheet is an outstanding question for understanding the connection between plasma sheet enhanced flows and substorm onset. Another question is the initiation of pre-onset PBIs, which mark the beginning of the pre-onset auroral sequence. We addressed these issues using THEMIS imagers and spacecraft, and ground-based radars.
Observations
Two-dimensional l-o-s flow distributions and auroral observations are shown in Figure 1 to display the relation between the polar cap flows and PBIs. A flow burst occurred in the central meridian of the RANK radar at 06:08 UT, clearly showing a longitudinally narrow region of enhanced equatorward flows in the polar cap extending down to the poleward boundary of the auroral oval. The PBI occurred ~1 min later at the same longitude or slightly to the west of the enhanced flow channel. Furthermore, the MLT widths of the flow burst and PBI are about the same.
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| Figure 1. Comparison between the SuperDARN l-o-s velocity and the auroral keogram at Rankin Inlet. The latitudinal profiles of the flow were obtained from Beams 0 and 1 averaging (westward looking), 7 and 8 averaging (northward looking), and 10 (eastward looking). Positive is directed toward the radar. Vertical lines mark PBI initiation times. Flow bursts preceding or simultaneous with PBIs are marked by red arrows. |
| Click here to enlarge the image. |
Latitudinal profiles of the line-of-sight (l-o-s) velocity from the SuperDARN radar at RANK in comparison with a keogram covering the poleward boundary of the oval (Figure 2) show a number of flow bursts directed toward the radar can be identified as transient blue strips. These are longitudinally narrow, transient equatorward flow channels in the nightside polar cap. As indicated by the vertical lines and red arrows, the flow bursts tend to occur prior to or simultaneously with the PBIs to within the 1 min time resolution of the SuperDARN observations.
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| Figure 2. Spatial distribution of the SuperDARN l-o-s velocity (RANK and SAS) overlaid onto the ASI data (RANK and GILL). |
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The spatial correspondence between the PBIs and preceding flow bursts suggests that enhanced flows in the polar cap contribute to PBI triggering, including those PBIs which mark the beginning of the auroral sequence leading to substorm onset. The spatial and temporal association of the flow bursts and PBIs suggests that the flow bursts in the entire length of the separatrix strengthen the parallel potential drop at the low altitude magnetosphere, leading to reconnection triggering.
Figures 3c and e represent intensity variations of N-S arcs and growth phase arc, respectively. Four N-S arcs 1-4 can be seen as rapid increases in intensity followed by equatorward motion. We have not found an identifiable difference in the intensity, propagation speed or spatial size between the N-S arcs 1-3 and N-S arc 4 that may account for why N-S arc 4 lead to a substorm and the others did not. On the other hand, intensity of the growth phase arc increases at the poleward portion of the pre-existing diffuse-like growth phase arc right after the N-S arc 3 reached this latitude and a thin arc formed. This arc developed into the onset arc soon after the N-S arc 4 reached the equatorward portion of the auroral oval, when the growth phase arc was much brighter than that at the times of the previous N-S arcs.
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| Figure 3. Line plots of the auroral intensity at 67.1 deg MLAT from Panel b (Panel c) and 64.7 deg (red) and 65.6 deg (blue) MLAT from Panel d (Panel e). The auroral keograms and magnetograms are the same as in Figure 1. The vertical lines only in Panels b and c mark the arrival of the N-S arcs. The vertical lines only in Panels d and e indicates growth phase arc intensifications. The last vertical line marks the substorm onset time. |
| Click here to enlarge the image. |
Conclusion
By taking an advantage of the SuperDARN two-dimensional observations coordinated with auroral imaging, we have shown for the first event that the observed PBIs were preceded by polar cap flow bursts and that the flow bursts are found to have narrow longitudinal widths and short durations. The longitudinal extent of PBIs has a correspondence with those of the flow bursts. These results lead us to suggest that enhanced flows in the polar cap may contribute to parallel potential drop increase at the low altitude magnetosphere. The reconnection in the magnetotail is also inferred to be triggered when the flow bursts reach the plasma sheet, based on the following equatorward-moving N-S arcs. Thus, the flow bursts on open field lines initiate the auroral time sequence leading to substorm onset.
Multiple equatorward-moving N-S arcs originated from the PBIs and moved toward the growth phase arc. However, only one of these led to the onset, and we did not find any significant difference in N-S arc features. On the other hand, all of the N-S arcs were found to be related to intensification of a narrow arc along the poleward boundary of the pre-existing diffuse-like growth phase arc. For both of the events, the growth phase arc at the time of the last N-S arc before the onset was much brighter than at preceding times. This difference leads us to suggest that the near-Earth plasma condition was not sufficient for substorm onset instability at the times of the preceding N-S arcs, and in particular that the plasma pressure in the near-Earth plasma sheet was not sufficiently large until the time period of the onset-related N-S arc.
Source
Biographical Note
Toshi Nishimura is an Assistant Researcher in the Department of Atmospheric and Oceanic Sciences at UCLA, and also a JSPS research fellow at the Nagoya University. His research interest is magnetosphere-ionosphere coupling during storms and substorms.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu